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IFB, Hops and Talent RX question...


Jamesg

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On bag based jobs where I have a couple of talent receivers (Sometimes Lectro, sometimes Zax) as well as hop and IFB transmitters (all SMQV's) I usually keep the talent wireless to lower blocks (19 or 20) and the hop and IFB transmitters in higher blocks (24 and 25). So my question is:

If I were to have the camera hop TX on block 19, the talent receivers on block 22, and the IFB's on 24, am I theoretically likely to hit more intermod problems by effectively "sandwiching" my receiver blocks in between two transmitting blocks? There will still be a block of separation between transmitting and receiving blocks, but my gut says that spreading the frequencies out across the spectrum can't be the best idea...

One thing to note - the job is an adventure type show, so the spectrum is generally pretty open, so I'm not dealing with too much other RF.

Obviously I'll do some tests, but in the meantime, any thoughts? J.

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In my opinion, and I am not an RF engineer, it is better to have the talent tx in lower blocks than the ifb and hop.
Harmonics from the tx frequencies can cause problems with tx on higher frequencies so it makes sense to put less essential tx there.
I have to admit though, that I don't know how many harmonics current tx actually still create.
I think that spreading out frequencies is actually a very good idea

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Depends on space between the antennas (transmitters and receivers), rf power (of hop and IFB, lower better in this case), intermod calculation and the filters in your receivers (e.g. 411 are good). Improving any of these values will increase talent's range dramatically.

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Totally, and I do all the usual scanning and freq finder stuff - I'm wondering specifically if having my talent receiver block in between the IFB and hop blocks, frequency wise, is likely to cause more issues than having the transmitters grouped into one block at one end of the spectrum and the receivers grouped into one block at the other end of the frequency spectrum. I'll still have at least one clear block in between the blocks I'm using... 

18 hours ago, Constantin said:

In my opinion, and I am not an RF engineer, it is better to have the talent tx in lower blocks than the ifb and hop.
Harmonics from the tx frequencies can cause problems with tx on higher frequencies so it makes sense to put less essential tx there.
I have to admit though, that I don't know how many harmonics current tx actually still create.
I think that spreading out frequencies is actually a very good idea

That's always been my workflow too, but I wonder if that's only true for adjacent blocks? I'll run some tests and see what happens...

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Totally, and I do all the usual scanning and freq finder stuff - I'm wondering specifically if having my talent receiver block in between the IFB and hop blocks, frequency wise, is likely to cause more issues than having the transmitters grouped into one block at one end of the spectrum and the receivers grouped into one block at the other end of the frequency spectrum. I'll still have at least one clear block in between the blocks I'm using... 
That's always been my workflow too, but I wonder if that's only true for adjacent blocks? I'll run some tests and see what happens...

Well, if there are harmonics, they'll be spread across the entire spectrum, with your chosen frquency as the fundamental, the lowest frequency. Of course, the harmonics loose power very soon, so it's unclear how much damage they can do. But these harmonics and your tx frequency can cause intermod, too, which can affect lower frequencies, too. I don't know how much of an issue that is in real life, with all these tracking filters and highly refined transmitters and receivers, but I would still say that sandwiching your important talent tx in between your not so important hop and ifb is a bad idea. Keep them as far apart as possible, with your talent tx on the lower freqs.
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I just got round to doing a test... I don't have the exact system I'd be using, but I set up two TRX-LA3.5 on block 22 @100mw, 3x wisycom tx on block 19 @50mw, and 2x SMQV on block 24 @100mw... The trx's as talent mics, received by a QRX200 with whip antennas in the bag, and all the other transmitters right next to the QRX in the bag too, antennas almost touching (I was trying to go for worst case scenario)...

Freqs coordinated on Freqfinder.

Walked the TRX's to the edge of their range with all the transmitters in the bag switched off, then turned them all on one by one. Absolutely no reduction in range at all.

Obviously I'm mixing digital with analog here, so it's possible that if everything was analog (or hybrid) it might be a different story, but it looks like all is well in this case.

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Just got back to my laptop from a trip so here's some comments: 
As far as harmonics of transmitters: If a transmitter is at 500 MHz the harmonics will be at multiples of 500 MHz, i.e., 1000 MHz, 1500 MHz, etc. These are so far from the receiver passband, they should cause little of no problems plus the transmitters output filters knock these multiples way down or the regulatory agencies get unhappy. Don't worry about harmonics in your own setup. They could interfere with higher frequency gear such as modems.

As far as using receivers in between two blocks: The intermod frequencies that are the worst are third order. They have the mathematical propery of always being below the lowest of the two transmitters and above the higher of the two transmitters. If the transmitters are at  500 and 600 MHz, the 3rd intermod products will be at 400 MHz and 700 MHz (subtract the transmitter frequencies and then add that difference to the higher transmitter and subtract that difference from the lower transmitter).  So, there are no strong intermod frequencies in between the lower and higher blocks of frequencies.

So far, we have eliminated harmonics and intermod as a problem for between two block operation. There are other considerations such as spurs from local oscillators, power supplies and modulation noise that are close to the transmitter carriers. They can be a few kHz to 10 MHz . Digital transmitters commonly have relatively high levels of noise close to the carrier. If you are operating between blocks, you could be forced to be close to a carrier and this might be a problem.  If you have good separation and clean transmitters, operating between blocks is no worse than operation above or below the two blocks. My recommendation, when you have down time, is to set up a test situation with transmitters at real world distances, both near and far, and make sure everything operates the way you think it should.

Best Regards,
Larry Fisher

 

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Wow - now THAT is an answer! Thank you so much!

That explains why in my test, which was really a worst case scenario with the transmitters and receivers much closer than they would be out in the wild, I didn't see any difference in reception when the transmitters were switched on or off. As I mentioned, I'm using this particular setup in remote locations so I'm not likely to hit any unknown frequencies, and I can coordinate everything. I can see how one could end up getting effectively "boxed in" if you were to do this in a frequency rich environment, though... J.

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